Crystallization and Solid-State Structure of Poly(l‑2-hydroxy-3-methylbutanoic acid)

The side-chain-substituted poly­(lactic acid)­s (substituted PLAs) have attracted much attention as novel bio-based polymers, but their possibility as crystalline polymeric materials still remains unknown. In this study, the crystallization behavior and solid-state structure are comprehensively investigated for poly­(l-2-hydroxy-3-methylbutanoic acid) [P­(L-2H3MB)] with <i>M</i>_n = 6.2 × 10⁴, i.e., stereoregular substituted PLA having isopropyl side chains. The equilibrium melting temperature of P­(L-2H3MB) is 240 °C, which is significantly higher than that of poly­(l-lactic acid) (PLLA) (≈200 °C). In addition, the isothermal and nonisothermal crystallization behaviors show that the crystallization of P­(L-2H3MB) is extremely faster than that of PLLA. X-ray diffraction results suggest that P­(L-2H3MB) has two or more crystal modifications, but only one modification appears with a high crystallinity (≈60%) when melt-crystallized at 155–180 °C

Complex Crystal Formation of Poly(l-lactide) with Solvent Molecules

By screening examinations for a wide variety of organic solvents, we found that poly­(l-lactide) (PLLA) forms the crystalline complex (ε-form) with the specific organic solvents such as tetrahydrofuran (THF) and <i>N</i>,<i>N</i>-dimethylformamide (DMF) below room temperature. It was revealed that PLLA has high selectivity for low molecular weight compounds to form the ε-crystals. By fiber diagram analyses for the ε-forms, it was found that PLLA chains take the 10₇ (left-handed 10₃) helical conformation and are packed in the orthorhombic lattice (<i>a</i> = 1.5–1.6 nm, <i>b</i> = 1.2–1.3 nm, <i>c</i> = 2.8–2.9 nm, and α = β = γ = 90°). Based on <i>R</i>-factor and packing energy calculations, the plausible crystal structure of PLLA–DMF complex was proposed, in which four PLLA chains and eight guest solvents are packed in the unit cell

Guest-Induced Crystal-to-Crystal Transitions of Poly(l‑lactide) Complexes

In this study, we systematically investigated various crystal-to-crystal transitions relating to poly­(l-lactide) (PLLA) cocrystallized with low-molecular-weight compounds using wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. First, the solvent exchange and the resultant crystal transition of solvent complexes were investigated. Basically, the solvent exchange treatments at −25 °C became successful, although some specific phenomena depending on solvent species were seen. The ratio of the α-form in the crystalline region increased by an increase in the solvent exchange temperature. Second, the crystal transition behavior between CO₂ and solvent complexes was investigated. The complete transition from solvent complex to CO₂ complex was observed for all solvent complexes. In contrast, it was found that types of solvents and the surrounding temperature have a great effect on the transition behavior from CO₂ to solvent complexes. Finally, the guest-induced transitions of noncomplex crystals (α-, α′-, and α″-forms) were examined. As a result, it was revealed that the guest-induced transition behavior of noncomplex crystals was much influenced by the order of crystal (chain conformation and packing) of noncomplexes (α, α′, and α″) as well as kinds of guests

Isothermal Crystallization Kinetics of Poly(ε-caprolactone) Blocks Confined in Cylindrical Microdomain Structures as a Function of Confinement Size and Molecular Weight

The isothermal crystallization kinetics of poly­(ε-caprolactone) (PCL) blocks confined in cylindrical microdomain structures (nanocylinders) formed by the microphase separation of PCL-<i>block</i>-polystyrene (PCL-<i>b</i>-PS) copolymers were examined as a function of nanocylinder diameter <i>D</i> and molecular weight of PCL blocks <i>M</i>_n. Small amounts of polystyrene oligomers (PSO) were gradually added to PCL blocks in PCL-<i>b</i>-PS to achieve small and continuous decreases in <i>D</i>. The time evolution of PCL crystallinity during isothermal crystallization at −42 °C showed a first-order kinetic process with no induction time for all the samples investigated, indicating that homogeneous nucleation controlled the crystallization process of confined PCL blocks. The half-time of crystallization <i>t</i>_1/2 (inversely proportional to the crystallization rate) of PCL blocks with <i>M</i>_n ∼ 14 000 g/mol showed a 140-fold increase (from 0.48 to 69 min) by a 16% decrease in <i>D</i> (from 18.6 to 15.6 nm). Another set of PCL-<i>b</i>-PS/PSO blends involving slightly longer PCL blocks with <i>M</i>_n ∼ 15 800 g/mol showed a similar result. It was found by combining the results of two PCL-<i>b</i>-PS/PSO blends that the small increase in <i>M</i>_n (from 14 000 to 15 800 g/mol) yielded an approximately 90-fold increase in <i>t</i>_1/2 (from 0.76 to 67 min) for PCL blocks confined in the nanocylinder with <i>D</i> = 18.2 nm. It is possible from these experimental results to understand the individual contributions of <i>D</i> and <i>M</i>_n to the crystallization rate of block chains confined in nanocylinders

Crystal Polymorphism of Curdlan Propionate: 6‑Fold versus 5‑Fold Helices

The molecular and crystal structures of curdlan propionate (CDPr) were examined by the X-ray fiber diffraction methods combined with energy calculations. CDPr forms two different crystal structures (CDPr forms I and II) depending on annealing conditions: solvent-annealing yields CDPr form I, whereas thermal-annealing gives CDPr form II. In CDPr form I, the 6/1 helix is packed in the hexagonal unit cell with <i>a</i> = <i>b</i> = 1.154 nm, and <i>c</i> (fiber axis) = 2.287 nm. In the case of CDPr form II, the 5/1 helix is packed in the pseudohexagonal cell with <i>a</i> = <i>b</i> = 1.175 nm, and <i>c</i> (fiber axis) = 1.859 nm. The crystal transition from CDPr forms I to II occurs by thermal-annealing at temperatures ≥ 160 °C